1. Field of the Invention
This invention relates to the field of medicine in which optical pulses are used to treat human tissue, including for example laser dermatology, laser surgery, and laser medicine. More specifically, this invention relates to controlling the optical pulses used in the treatment.
2. Description of the Related Art
The use of optical pulses to treat human tissue is now quite widespread. In these procedures, optical pulses from a suitable optical source, such as a laser, a lamp, or an electromagnetic pulse generator, are directed at human tissue. The optical pulses typically pass through the surface of the tissue and into the subsurface layers of the tissue, where beneficial changes occur in the tissue in response to the passage of the optical pulse.
A common challenge facing practitioners of this type of procedure in vivo is the variation in tissue response from one treatment to the next. Variations may be the result of differences between individuals. Differences between patients may exist for many reasons, including being associated with the general health, age, sex, race and/or the history of sun exposure. Variations may also be the result of differences in tissue type and/or responsivity within a single individual. Trends may be seen where the response of the tissue depends on the presence of bioagents in the tissue, such as melanin, hemoglobin, interstitial fluid, fatty matter, or foreign matter such as tattoo dyes. For example, in one common technique, optical pulses in the wavelength range 400-800 nm are used to heat the tissue below the surface of the human skin, which exhibits change as a result of the elevated temperature. The amount of melanin in the skin greatly affects the penetration and heating effect of the optical pulses. Often the practitioner proceeds without knowing precisely what the responsiveness of the tissue will be. As a result, there is an increased risk that the tissue will be over- or under-treated.
Accordingly, a significant concern for practitioners is the correct selection of the parameters of the optical pulses used to irradiate the tissue. These parameters can include, but are not limited to, the pulse energy, spot size, wavelength, temporal pulse shape, spatial pulse shape, pulse duration, focus location, depth of focus, optical polarization, or angle of incidence on the treatment region. These parameters can also include aggregate quantities, such as the number or density of optical pulses directed to a site or the total energy deposited at a site. Pulse parameters such as the spacing between pulses, the separation between sequentially irradiated sites, and the shape of a sequence of pulses can also be adjusted. For each treatment, the parameters ideally should be selected to produce the desired beneficial effect, taking into account all variations regardless of the cause. The concern arises from the significant variation in tissue response. Irradiation parameters that produce no effect in one case may possibly produce a damaging effect in another. Thus, the effectiveness of the treatment often depends of the ability of the practitioner to judge or otherwise determine the appropriate pulse parameters before treatment.
The process of detecting the tissue response, and thereby determining when the treatment should be terminated, is often referred to as “end-point detection.” Methods to detect tissue response to a single pulse in real-time have been suggested, but many methods are unsuitable for end-point detection or other types of control, being either too inaccurate or too slow to give timely guidance to the practitioner. For example, in laser dermatology using laser pulses of 1 millisecond pulse length, the time available to diagnose the tissue response is only a few hundred microseconds. Few, if any, techniques are suitable for end-point detection of subsurface tissue response in this short time frame.
Another method sometimes used in current practice is to treat a small, relatively unnoticeable portion of the tissue to test the optical pulse parameters. Another method simply relies on the practitioner's skills of observation, experience and judgment. Yet another method makes measurements of related optical properties of the tissue immediately prior to treatment, and this data is used in combination with the practitioner's judgment, skills and experience to set the pulse parameters. These methods have a variable level of reliability because they are inexact and do not always account for variables that can significantly affect the pulse parameters. Even treatments of the same area of the same patient's tissue by the same practitioner on different occasions may require different optical pulse parameters. As another example, pre-testing small regions of tissue may require several attempts before the correct settings are determined, and, to the extent that the tested regions are located at some distance from or are otherwise different from the actual region to be treated, the results might be inaccurate.
Thus, there is a need for systems and methods for real-time control of optical pulses used to treat tissue.